This invention relates to systems and methods for light beam scanning and display, and more particularly to such systems and methods as applied to large screen video displays of wide bandwidth.
The patent and technical literature contain many references to TV display systems using laser beam excitation. Much of the literature is concerned with the problem of beam modulation, as in U.S. Pat. No. 3,691,484 and in an article entitled "A Television Display Using Acoustic Deflection and Modulation of Coherent Light", by A. Korpel et al, published in APPLIED OPTICS, Vol. 5, p. 1667, October 1966. This literature discusses other factors as well, including scanning techniques, which are more comprehensively reviewed in an article entitled "Laser Display Technology", by Charles E. Baker, printed in the IEEE SPECTRUM, Vol. 5, No. 12, December 1968, pp. 39-50. The latter publication has some discussion of the numerous causes of energy attenuation in the system subsequent to the laser, reaching the conclusion that the "low efficiency of presently available lasers prevents any serious consideration of competing with CRT displays in the immediate future..." and that "Further application of laser display technology rests on the development of a practical, low-cost laser with an efficiency exceeding 1%".
Losses occur not only in the laser and modulating elements, such as the acousto-optical elements typically used, but also in the scanning and display portions of the system. The useful output of a system is to be measured in terms of the visual information that may be perceived by an observer. In a video or other wide bandwidth display, whether color or monochrome, the discernible information is a function of resolution and contrast as well as light intensity. In a color spot scanning system, for example, color purity, spot resolution, and the light contrast between the illuminating beam and the background can be more important than mere brightness. Despite the low efficiency of lasers it is desirable to provide displays approaching or equal to presently available television displays, in terms of derivable information, without employing large or high powered lasers. Thus maximum advantage must be taken of the energy available in the beam in terms that are useful to the viewer, in addition to minimization of attenuation. This becomes of great importance with large screen displays, because of the decrease in illumination within a given incremental area as total scan area is increased. At the same time, however, color purity and tonality must be consistent with present high standards. In order to achieve results closer to idealized color characteristics, most workers in the art have started with the assumption that it was necessary to utilize separate lasers, each generating a particular red, green or blue wavelength. Attempts have also been made to utilize the persistence and excitation characteristics of phosphors, as in U.S. Pat. Nos. 3,652,956 and 3,760,096, in order to achieve improved results. However, substantial losses occur in phosphor excitation, and both the lowered efficiency and increased complexity outweigh the benefits derived in terms of color display.
The literature also reveals that much work has been done on mechanical, electromechanical and electronic scanning systems for laser TV displays. Significant problems are encountered in the horizontal scanning motion, because of the problems involved in obtaining a scanning rate of 15,734 Hz, in accordance with U.S. standards. As described in the literature, acousto-optical scanners, fiber optic devices and various other light transmitting elements can be operated at such frequencies, but at the expense of substantial beam attenuation and some beam dispersion. While the laser is ordinarily visualized as providing a concentrated light beam, the beam actually has a meaningful cross sectional area and a Gaussian distribution of light intensity across the area. The use of many light transmissive elements can markedly decrease maximum beam intensity, increase beam area, and generate spurious beams.
Electromechanical beam scanners, such as small mirrors driven by galvanometer movements, have limitations both as to response time and linearity. Ideally, the scanned beam should move linearly with time over the display area. With a high horizontal scan rate, such as is used in a television display, significant limitations are encountered in these respects when scan angles are large. In order to illuminate a large screen display, therefore, the beam envelope must be expanded significantly along a relatively longer path length. By "large screen display" is meant a display larger than those achievable with modern direct display television sets, such as the common 25 inches diagonal cathode ray tube systems, and including typical sizes of 3 feet .times. 4 feet, 6 feet .times. 8 feet, and considerably larger areas, such as 15 feet .times. 20 feet displays.
It is feasible, of course, to incorporate a scanning laser in a direct projection system, with the laser projector and the screen being separated by some predetermined distance. Practically all present large screen systems, such as cathode ray tube projection systems and theater TV-type installations based on the Eidophor principle, use direct screen projection. The direct projection system is, however, often unduly cumbersome and inconvenient, and existing systems have difficulty in maintaining focus and freedom from distortion across the image area. Furthermore, a non-scanning laser beam of more than moderate energy levels can be hazardous. It is far preferable for most applications to have a single large screen display structure of shallow depth, and especially for laser systems to maintain the entire beam path within a closed structure. Typically, the depth dimension should be less than the transverse dimension of the display face. For aesthetic and installation purposes, it will often be preferred that the depth dimension be as small as possible, and a fraction of the dimensions of the display.
The large screen video display merely represents one example of a wide bandwidth display system for which a need exists. The principles and practical exemplifications of a large screen laser generated display of high useful display information output can be employed in monochromatic displays, lower bandwidth systems and specialized systems. They can be applied to visual displays of all kinds in which electron beams have traditionally been employed to generate images, especially conventional closed circuit displays such as are used with data processor and communications systems. The uses may be extended to thermograms, fluoroscopy and other contexts involving pseudocolor (conversion of intensity differences in parts of a transmitted image to differences in color). Such displays should be achievable with presently available lasers, and particularly with lasers that are economically realistic for a given use. That is, neither the initial cost nor the power requirements of the laser should be excessive, in terms of the particular domestic or industrial application for which it is being employed.